Introduction to Carbohydrates:

Carbohydrates are a class of organic compounds made up of carbon, hydrogen, and oxygen atoms. They are crucial biomolecules found in both plants and animals. Carbohydrates are primarily responsible for providing energy in the form of calories and are essential for various biological functions. These compounds are formed during photosynthesis in plants and are consumed by both plants and animals for fuel.

• Empirical Formula: The general empirical formula of carbohydrates is CnH2nOn or (CH2O)n, which indicates a combination of carbon and water.
• Occurrence: Carbohydrates account for 2–40% of the total mass in plant tissues, with the lowest levels found in cucurbits (such as cucumbers) and the highest in cassava.

Carbohydrate Functions in the Body:

1. Energy Supply: The primary function of carbohydrates is to provide energy. Every gram of carbohydrate provides 4 kcal/g. In the Indian diet, carbohydrates contribute about 65-80% of the total energy.
2. Reserve Fuel Supply (Glycogen): Carbohydrates serve as a reserve fuel in the form of glycogen, which is stored in the muscles and liver. The total glycogen in the body is approximately 300 grams, but it needs to be replenished regularly through carbohydrate intake to prevent the breakdown of fat and proteins for energy.
3. Structural Functions: Carbohydrates provide the chemical framework needed for the synthesis of non-essential amino acids (amino acids not required in the diet) by combining with nitrogen.
4. Metabolic Compounds: Carbohydrates and their derivatives play a vital role as precursors for important metabolic compounds, such as:
   • • Nucleic acids (DNA and RNA)
     • Galactosides of nerve tissue
     • The matrix of connective tissues
5. Brain Development: Lactose (milk sugar), a carbohydrate found in milk, provides galactose, which is essential for brain development. It also aids in the absorption of calcium and phosphorus, promoting bone growth and maintenance.
6. Gut Health: Lactose also promotes the formation of lactic acid in the intestines due to the action of bacteria (lactobacilli), which synthesizes some B-complex vitamins. This helps in suppressing harmful bacteria (putrefactive bacteria) and promoting gut health.
7. Immune Function: Certain carbohydrate derivatives are involved in immune responses. For example, immunopolysaccharides increase the body’s resistance to infections.
8. Metabolism of Fats: Carbohydrates are essential for the normal metabolism of fats, preventing acidosis (an accumulation of acid in the body).
9. Dehydration Prevention: A low carbohydrate diet can cause dehydration by depleting water and electrolytes (sodium and potassium) in tissues.
10. Dietary Fiber: a type of carbohydrate, acts like a sponge, absorbing water and aiding in the smooth movement of food waste through the digestive tract. It helps in eliminating soft, bulky stools.

Food Sources of Carbohydrates: Carbohydrates are primarily found in plant-based foods, and the main forms include starch, sugars, and dietary fiber.

1. Starch: Found in cereals, legumes, roots, and tubers (e.g., potatoes, cassava, yams). These provide a major part of energy in the diet.
2. Complex Carbohydrates in Indian Diet: Complex carbohydrates are commonly consumed in the form of cereals (e.g., wheat, rice), dals, and legumes in India, providing energy, proteins, and vitamins.
3. Sugars in Fruits: Fruits contain simple sugars like glucose, fructose, and sucrose. For example:
   • • Melons contain about 3% sugar.
     • Citrus fruits and guavas contain around 10% sugar.
     • Mangoes can contain up to 17% sugar.
   • Sugars in fruits are quickly available sources of energy.
4. Milk (Animal Source): The primary carbohydrate in milk is lactose, an important source of energy for infants and also crucial for calcium absorption, aiding bone development.
5. Other Sources of Sugars: Sugarcane and beetroot are the sources of sucrose, which is extracted and purified into table sugar.

Classification of Carbohydrates:

1. Monosaccharides: These are simple carbohydrates with 3 to 8 carbon atoms. Glucose and fructose are the most common monosaccharides in food, both with the formula C6H12O6. Monosaccharides can be divided into aldoses (e.g., glucose, with an aldehyde group) and ketoses (e.g., fructose, with a ketone group).
2. Oligosaccharides:
   • These are carbohydrates containing 2 to 10 monosaccharides linked by glycosidic bonds.
     • Disaccharides (two monosaccharides):
       • Lactose (milk sugar) = Glucose + Galactose
       • Maltose (grain sugar) = Glucose + Glucose
       • Sucrose (table sugar) = Glucose + Fructose
     • Trisaccharides:
       • Raffinose = Fructose + Glucose + Galactose
3. Polysaccharides: These are complex carbohydrates made up of more than 10 monosaccharide units. Examples include starch (found in cereals and tubers), cellulose (the structural component of plant cell walls), and glycogen (the storage form of glucose in animals). Polysaccharides can be homoglycans (contain one type of sugar) or heteroglycans (contain different types of sugars).

Reducing vs. Non-Reducing Sugars:

• Reducing sugars: These have a free aldehyde or ketone group and can reduce other compounds. Examples include lactose, maltose, and all monosaccharides.
• Non-reducing sugars: These do not have a free aldehyde or ketone group and cannot reduce other compounds. An example is sucrose.

Sugars in Fruits and Vegetables:

• Tropical and subtropical fruits generally have the highest sugar levels. For example, grapes are rich in sugar.
• Beetroot is one of the vegetables with the highest sugar content, mainly sucrose.

Sweetness Value of Sugars: Different sugars have varying levels of sweetness:

• Fructose is the sweetest, with a sweetness value of 173.
• Sucrose (table sugar) has a sweetness value of 100.
• Glucose has a lower sweetness value of 74.

Fiber in Carbohydrates: Dietary fiber, which is part of carbohydrates, includes substances like cellulose, pectins, and hemicelluloses. These provide bulk to food and are essential for digestive health. They absorb water, regulate bowel movements, and reduce constipation.

Carbohydrates in Cereal:

• Cereals are rich in carbohydrates, primarily in the form of starch.
• Fiber content varies among different cereals, and it plays an important role in digestion.
• The crude fiber content of cereals can range from 0.5% in brown rice to 10.9% in oats.
• Starch in cereals consists mainly of amylopectin (74-79%) and amylose (25-30%).

Organic Acids in Fruits and Vegetables: Fruits and vegetables also contain organic acids, which contribute to their taste (sourness):

• Malic acid: Found in apples, bananas, watermelon, and cherries.
• Tartaric acid: Found in grapes and tamarind.
• Citric acid: Found in citrus fruits (e.g., oranges, lemons) and pineapples.
• Oxalic acid: Found in tomatoes.